|Trouble Code||Fault Location||Probable Cause|
|P0156||Heated oxygen sensor (H025) 2, bank 2 circuit malfunction||Heating inoperative, wiring, H028, ECM|
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What Does Code P0156 Mean?
OBD II fault code P0156 is defined as “Oxygen Sensor Circuit Malfunction (Bank 2 Sensor 2)”, and is set when the PCM (Powertrain Control Module) detects a voltage or signal from or to the #2-oxygen sensor that does not fall within predefined specifications. In this definition, “Bank 2” refers to the bank of cylinders that does not contain cylinder #1, while “Sensor #2” refers to the oxygen sensor that is located downstream, or after the catalytic converter.
An electrical resistance of about 8 – 12 Ohms is typical for this circuit, and a deviation of around 10% to either side of this value will set code P0156 and illuminate a warning light. Note however that these values may differ from one application to the next: always consult the manual for the application being worked on for the exact code setting parameters for that particular application. Note also that some applications require several fault cycles to occur before a warning light will be triggered.
The distinction between the functions of the oxygen sensors located before and after the catalytic converter(s) is an important one. While sensor #1 (located before the converter) is primarily concerned with analyzing the exhaust stream to detect changes in the level of oxygen in the gas so that changes to short-term fuel trim can be made by the PCM, the #2 sensor (located after the converter), is concerned with monitoring the function of the catalytic converter.
In practical terms, a fully functional #2 oxygen sensor fulfills a diagnostic role, and once it has entered closed loop operation, which is when its heating element has reached its proper operating temperature of around 316 °C (600 °F), it should deliver a relatively steady signal voltage of between 0.5 volt, and 0.7 volt. Major fluctuations in this signal voltage indicate a defective catalytic converter, since changes in the composition if the exhaust gas are not being absorbed, or “damped out” by the converter.
Conversely, signal voltages from the #2 sensor that fluctuate from much below about 0.5 volt to much above 0.7 volt, or a signal voltage that remains static without changing at all indicate a defective sensor, or a fault in the sensor’s control and/or signal circuit.
The image below shows the typical appearance of an oil-fouled oxygen sensor. Excessive oil burning is a common cause of oxygen sensor related codes, which is more common on applications that are known for high oil consumption rates.
What are the common causes of code P0156 ?
Common causes of code P0156 are much the same across all applications, and could include the following-
- Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors
- Open circuits
- Blown fuses (where applicable)
- Defective oxygen sensors
- Heavily fouled or contaminated oxygen sensors
- PCM failure is a rare event, and the fault must be sought elsewhere before any controller is replaced
What are the symptoms of code P0156 ?
Typical symptoms could include the following, but note that not all symptoms will always be present on all applications-
- Stored trouble code, which may or may not be accompanied by a warning light, since some applications require multiple fault cycles to occur before a warning light is triggered. In these cases, the code may be stored as a “pending” code.
- Rich running conditions may be present. In extreme cases, black smoke from the tail pipe may be present.
- Reduced fuel economy may be present.
- Lean running conditions may be present.
- Engine may hesitate or stumble upon acceleration.
- Rough idling may be present.
How do you troubleshoot code P0156 ?
NOTE #1: Diagnosing code P0156 requires that the engine be in perfect running condition. The fuel/air ratio must be delivered at the ideal 14.7: 1 ratio, there must be no misfiring codes present, and there must be no vacuum or exhaust leaks present. If any air/fuel metering codes are present along with P0156, these codes must be resolved in the order in which they were stored before attempting a diagnosis of this code. Failure to do this will almost certainly result in a misdiagnosis and many hours of wasted diagnostic time.
NOTE #2: Exhaust leaks have the potential to contaminate the ambient atmospheric air used by all oxygen sensors for reference purposes, which can result in false readings. Therefore, be sure that all exhaust leaks present are repaired before starting a diagnostic procedure for this code.
NOTE #3: Clear all codes and retest the system after each step in the diagnostic/repair procedure to ensure a successful repair.
Record all fault codes present, as well as all available freeze frame data. This information can be useful should an intermittent fault be diagnosed later on.
If no other codes are present and the engine is in perfect running condition, consult the manual to determine the location, function, routing, and color-coding of all wiring associated with the affected oxygen sensor. Perform a thorough visual inspection of all wiring (including fuses where applicable), and look for burnt, damaged, shorted, disconnected, or corroded wiring and/or connectors. Make repairs as required.
NOTE: It is common for wiring to burn, or short circuit against hot exhaust components. When making repairs to damaged wiring, be sure to route replacement wiring as far away from hot exhaust components as possible to prevent a recurrence of the problem.
If no damage to the wiring or connectors is found, connect the scanner to the diagnostic port, start the engine, and allow the system enough time to enter closed loop operation.
If the scanner can monitor live data streams, check all parameters of the affected sensor’s operation. Pay particular attention to the sensors’ temperature, which should be close to 6000 F (3160 C) as well as the value of generated signal voltage. When a downstream oxygen sensor is in closed loop operation, the signal voltage it generates should be at, or near the midpoint between a rich -, and lean running condition for that application.
This reading should remain within about 100-150 milli-volts of that point until the engine speed changes significantly; when the engine speed returns to idling, the displayed reading should return to the value it was at before the engine speed changed.
NOTE: Applications that are known for their high oil consumption rates such as BMW, Audi, VW, and Mercedes-Benz products, among others, can cause oil fouling of the upstream oxygen sensor and damage to the catalytic converter, which in turn, can cause the downstream (#2) sensor to produce misleading readings. On these applications, removal and inspection of affected oxygen sensors should be a first step in diagnosing code P0156.
If no damage to associated wiring is found, prepare to perform reference, input, and resistance, ground, and continuity tests on the wiring. Consult the manual on the correct procedure (KOER/KOEO) to establish a ground connection. Note however that on some applications, the ground is established through contact with the exhaust system. Also, be sure to disconnect the sensor from the PCM to avoid damage to the controller during resistance and continuity checks.
Compare all obtained readings with those stated in the manual, and replace the wiring harness of the affected sensor if discrepancies are found, since making repairs to wiring can result in high resistances and poor continuity.
NOTE: During this step, pay particular attention to both the sensor heater circuit wire, and the sensor signal wire leading to the PCM. Each sensor has its own dedicated heater control circuit, so check that the correct current for the heater element reaches the sensor at the connector, and that the resistance value of the signal wire matches the value stated in the manual exactly.
NOTE #2: On some applications, the input voltage for the sensor heater element is supplied by the PCM, in which cases the input voltage may be lower than battery voltage. Consult the manual for the correct heater control input circuit for the application being worked on.
If all electrical values fall within specifications, suspect a faulty oxygen sensor. Bear in mind that the sensor forms part of the control circuit, and as such, it must be tested as well. Consult the manual on the specified resistance value for the sensor, and check the resistance across the terminals. Replace the sensor if its internal resistance does not agree with the value stated in the manual.
NOTE: At this point, it may be necessary to remove the sensor from the vehicle to make testing easier. Also, note that as mentioned before, oil fouling or contamination by other substances in the fuel or engine oil can produce misleading readings, which can sometimes be interpreted by the PCM as circuit malfunctions.
With the sensor removed from the vehicle, inspect it for the presence of discoloration or the accumulation of deposits that could render the sensor inoperable. For instance, carbon deposits caused by excessive oil burning can cause internal short circuits, similar to the way carbon deposits can short circuit a spark plug.
Oil fouling of oxygen sensors on Audi, VW, Mercedes-Benz, and BMW products is a common problem. If it is certain that the excessive oil burning is not caused by an underlying mechanical issue that warrants repair, the only way to alleviate oxygen sensor issues on these applications is to replace the sensors on a regular basis.
Also, check for signs of contamination by anti-freeze and silicon-based compounds in some aftermarket fuel additives. The only remedy for this type of contamination of oxygen sensors is to repair the underlying problem in the case of anti-freeze contamination, and to cease the use of aftermarket fuel and oil additives. Simply replacing the oxygen sensor will not resolve the problem.
After repairs are complete, check to make sure that all wiring associated with the affected oxygen sensor is properly routed, and secured away from hot exhaust components to prevent possible damage to the newly repaired/replaced wiring.
Clear all codes, and operate the vehicle for at least one complete drive cycle with the scanner attached to monitor the operation of the replaced sensor and/or wiring in real time. If the code does not return after the completion of a several drive cycles, the repair can be seen as successful, but if the code does return, it is likely that an intermittent fault is present.
Note that intermittent faults can be extremely challenging to find and repair. In some cases, the fault may have to be allowed to worsen considerably before an accurate diagnosis and definitive repair can be made.
Codes Related to P0156
- P0136 – Relates to “O2 Sensor Circuit (Bank 1 Sensor 2)”
- P0137 – Relates to “O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2)”
- P0138 – Relates to “O2 Sensor Circuit High Voltage (Bank 1 Sensor 2)”
- P0139 – Relates to “O2 Sensor Circuit Slow Response (Bank 1 Sensor 2)”
- P0140 – Relates to “O2 Sensor Circuit No Activity Detected (Bank 1 Sensor 2)”
- P0141– Relates to “O2 Sensor Heater Circuit Malfunction (Bank 1 Sensor 2)”
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